Engine systems may utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system (intake passage), a process referred to as exhaust gas recirculation (EGR), to reduce regulated emissions. An EGR system may include various sensors to measure and/or control the EGR. As one example, the EGR system may include an intake gas constituent sensor, such as an oxygen sensor, which may be employed to measure oxygen to determine the proportion of combusted gases in an intake passage of the engine. The sensor may also be used during non-EGR conditions to determine the oxygen content of fresh intake air. The EGR system may additionally or optionally include an exhaust gas oxygen sensor coupled to the exhaust manifold for estimating a combustion air-fuel ratio.
As such, when the intake oxygen sensor is used for EGR control, the EGR is measured as a function of the change in oxygen due to EGR as a diluent. To determine the change in the amount of oxygen, a reference point corresponding to an oxygen reading when no EGR is present is required. Such a reference point is called the “zero point” of the oxygen sensor. Due to the sensitivity of the oxygen sensor to various conditions, such as pressure, aging, and piece-to-piece variability, there may be large deviations in the “zero point” at different engine operating conditions. Therefore the oxygen sensor may need to be regularly calibrated and a correction factor may need to be learned.
One example method for calibrating an exhaust gas oxygen sensor is depicted by Ishiguro et al. in U.S. Pat. No. 8,417,413. Therein, a correction factor is learned based on an oxygen sensor output during engine fuel-cut off conditions. However, the inventors have recognized that approaches used for zero point estimation in exhaust oxygen sensors may not be applied for zero point estimation of intake oxygen sensors. This is because in addition to being sensitive to pressure and part-to-part variability, due to equilibration of the sensed gas by a catalyzing sensing element of the sensor, the intake oxygen sensor is also sensitive to ambient humidity. Specifically, the water content of the intake aircharge may displace oxygen. If the reading is used to estimate EGR, more diluent may be estimated as the humidity increases. As a result, the measurement and/or control of EGR may be reduced.
In one example, some of the above issues may be addressed by a method for an engine comprising: estimating an ambient humidity while learning a reference point for an intake oxygen sensor at a reference intake pressure; and correcting the learned reference point based on the estimated ambient humidity. In this way, a zero point reading for an intake oxygen sensor may be corrected for the effect of varying ambient humidity, improving the accuracy of EGR control.
For example, at the first engine idle following every engine start, an idle adaptation of the intake oxygen sensor may be performed. Therein, an output of the intake oxygen sensor may be monitored, while also estimating a reference intake pressure (based on an intake manifold pressure sensor output) and an ambient humidity (based on an intake manifold humidity sensor output). The output of the intake oxygen sensor is corrected based on the estimated ambient humidity to learn a dry air nominal oxygen sensor reading. Alternatively, the output of the intake oxygen sensor is corrected based on the estimated ambient humidity to learn an oxygen sensor reading for a calibrated amount of humidity (e.g., for a pre-defined standard humidity level). A relationship between the corrected output of the intake oxygen sensor at the reference intake pressure may then be learned as the reference “zero point”. During subsequent engine non-idling conditions, a difference between an output of the intake oxygen sensor and the learned zero point may be used to estimate an EGR concentration, and thereby adjust an EGR flow.
In this way, the effect of humidity on the output an intake oxygen sensor can be compensated for. By measuring the ambient humidity at the time of learning the reference point of the intake oxygen sensor, the amount of oxygen displaced by the ambient humidity can be learned and used to correct the sensor output. By calibrating the sensor output to a dry air condition, where the effect of all humidity is removed, or to a standard air condition, where the effect of a standard humidity level is learned, the sensor output can be regulated to pre-defined conditions. By using the humidity-corrected zero point to estimate EGR flow, EGR calculation errors from variations in ambient humidity conditions can be reduced. Overall, the accuracy of EGR estimation is increased, allowing for improved EGR control.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.